Closed-loop precooling of cryogenically cooled equipment

ABSTRACT

Apparatus for pre-cooling cryogenically cooled apparatus housed within a cryogen vessel ( 26 ), comprising a first closed-loop cooling circuit ( 30 ) containing heat transfer fluid, a circulator ( 32 ) for causing the heat transfer fluid to circulate around the closed circuit and a heat extractor ( 34 ) arranged to extracts heat from the heat transfer fluid, wherein the circuit carries heat transfer fluid into and from an interior volume of the cryogen vessel ( 26 ).

The present invention relates to methods and apparatus for pre-coolingcryogenically cooled-equipment. In particular, it relates to suchcooling by a closed-loop refrigeration system. The present invention maybe particularly applied to the pre-cooling of superconducting magnetsfor MRI (magnetic resonance imaging) systems, but may of course beapplied to other cryogenically cooled equipment.

In typical current arrangements, apparatus to be cryogenically cooled ishoused within a cryogen vessel. The cryogen vessel is contained withinan outer vacuum chamber and the volume between the outer vacuum chamberand the cryogen vessel is evacuated, providing effective thermalinsulation. Pre-cooling of the apparatus is performed by simply addingliquid cryogen to the cryogen vessel and allowing it to boil off. Whileeffective, this arrangement has certain drawbacks.

If a working cryogen, such as liquid helium, is used for this pre-coolstep, the quantity of helium boiled off and vented to atmosphere iscostly, and it may be difficult to obtain sufficient supplies in someregions. Also, since liquid helium is a non-renewable resource, itsconsumption should be minimized where possible.

In certain arrangements, a sacrificial cryogen such as liquid nitrogenis initially used to cool the apparatus to a first temperature,typically higher than the temperature of the working cryogen. Once theequipment has been cooled to the first temperature by the sacrificialcryogen, a quantity of the working cryogen is added to cool theapparatus to the required temperature. The advantages of thisarrangement include the fact that an abundant, inexpensive sacrificialcryogen, such as liquid nitrogen, may be used as the sacrificialcryogen; and that the consumption of working cryogen is significantlyreduced below the consumption of the alternative arrangement where theworking cryogen is used alone. However, difficulties with this methodinclude the likelihood of contamination of the working cryogen withresidual quantities of sacrificial cryogen. If an amount of liquidnitrogen remains in a cryogen tank when liquid helium is added, asignificant quantity of liquid helium will be required to cool thenitrogen itself down to liquid helium temperature, cancelling some ofthe benefit of reduced helium consumption.

A complete flow-chart of a method for cooling cryogenically cooledapparatus according to the prior art is shown in FIG. 1.

The following description will be made with particular reference tosuperconducting magnets for MRI imaging apparatus, but it should beunderstood that the present invention may be suitably applied to thepre-cooling of any cryogenically cooled apparatus within a cryogenvessel.

In the first step 10, the cryogen vessel is evacuated and then filledwith helium gas at atmospheric pressure and ambient temperature. Thisenables the cryogen vessel to be tested for leaks. Any leakage of thehelium gas into the vacuum between the cryogen vessel and the outervacuum container typically provided around the cryogen vessel forthermal insulation may be easily detected.

In the second stage 12, the helium gas is flushed from the cryogenvessel, and pre-cooling is initiated by the addition of liquid nitrogen.The liquid nitrogen boils off to atmosphere as it cools the magnetstructure within the cryogen vessel. Liquid nitrogen has a relativelylarge thermal heat capacity and so is an effective coolant. It is alsoinexpensive, and so offers rapid and inexpensive cooling to a firstcryogenic temperature.

As shown at stage 14, the addition of liquid nitrogen continues until apredetermined quantity of liquid nitrogen remains in the cryogen vessel.

At step 16, the magnet is allowed to soak in the liquid nitrogen for acertain time, to allow the magnet structure to reach a consistenttemperature throughout, equal to the boiling point of nitrogen. Oncethis is complete, the liquid nitrogen is flushed from the cryogenvessel. This may be by the well known siphon effect, where helium gas atambient temperature is introduced into the cryogen vessel. Gas pressurein the cryogen vessel is used to force out liquid cryogen. Care must betaken to remove all, or as much as possible, of the nitrogen from thecryogen vessel. The cryogen vessel is then pumped out to vacuum toremove as much nitrogen as possible.

In the next step 18, liquid helium, or another working cryogen asrequired, is introduced into the cryogen vessel. The working cryogenboils off to cool the magnet to the required operating temperature.Working cryogen is added until a required quantity of working cryogenremains in the cryogen vessel.

Finally, at step 20, the magnet structure is at the requiredtemperature, and contains the required quantity of working cryogen.

While effective, this method still consumes a large quantity ofsacrificial and working cryogens. In one known system, with the magnetcooled to 70K by a liquid nitrogen sacrificial cryogen, 1200 litres ofliquid helium are consumed in cooling the structure from 70K to 4K. Ifthe nitrogen is not completely removed, there is a significant increasein the amount of helium required, since the remaining liquid nitrogenmust be frozen and cooled to the liquid helium temperature. If anycryogen remains within the cryogen vessel, it may act as a ‘poison’, inthat it may act to form an ice around the superconducting magnet coils,which in turn may cause the superconducting magnet coils to quench inoperation.

Prior art precool arrangements are described, for example, in EP1586833,U.S. Pat. No. 5,187,938, US2005/016187 and GB1324402. In thearrangements disclosed in both US20051016187 and U.S. Pat. No.5,187,938, a closed loop cooling circuit is used wherein the circulatingheat transfer material is cooled by a tank of liquid cryogen, such asnitrogen, but is warmed up to room temperature before passing through acirculator. This warming wastes any cooling power remaining in the heattransfer material and causes significant inefficiencies in the system.In U.S. Pat. No. 5,187,938, the heat transfer material is pressurisedslightly in excess of atmospheric pressure to prevent externalcontaminants from leaking in.

The present invention accordingly aims to alleviate at least some of thedisadvantages of the prior art. For example, a desire to reduce thevolume of helium required, and to remove risks associated with theintroduction of nitrogen into the cryogen vessel. It also aims tosimplify the pre-cool procedure. By using only one type of cryogenwithin the cryogen vessel, the need for repeated evacuations is avoided.

The present invention provides cooling apparatus in which it is notnecessary to warm the heat transfer material to room temperature beforepassing through the circulator. This significantly increases theefficiency of the proposed system. In preferred embodiments, the presentinvention also provides pressurised heat transfer material, pressurisedsignificantly above atmospheric pressure to improve the effectiveness ofheat transport.

Accordingly, the present invention provides methods and apparatus as setout in the appended claims.

The above, and further, objects, characteristics and advantages of thepresent invention will become more apparent from consideration of thefollowing description of certain embodiments; given by way of examplesonly, in conjunction with the accompanying drawing, wherein

FIG. 1 illustrates a flow chart of a conventional pre-cooling method forapparatus cooled to liquid helium temperature;

FIG. 2 shows a schematic diagram of a first embodiment of the presentinvention; and

FIG. 3 shows a schematic diagram of a second embodiment of the presentinvention.

According to the present invention, the current open-loop coolingmethod, based on the boiling of a liquefied cryogen by contact with thecooled apparatus, is replaced by a closed-loop pre-cooling arrangement.A coolant is circulated between the magnet, or other apparatus to becooled as appropriate, and a cold source. The cold source may be anactive refrigerator, or may be a boiling tank of liquid cryogen, acooled tank of liquid cryogen, or a frozen block of solid cryogen.

FIG. 2 shows a schematic diagram of a first embodiment of the presentinvention. In FIG. 2, a superconducting magnet structure 20, comprisingcoils 22 of superconducting wire wound onto a former 24, is shown housedwithin a cryogen vessel 26, itself housed within an outer vacuumcontainer 28. Such arrangement is entirely conventional, and may besubstituted for any other cryogenically cooled apparatus, as theapplication requires.

In accordance with the present invention, a closed-loop cooling circuit30 is provided. The cooling loop comprises a closed circuit-containingheat transfer fluid, a circulator 32 such as a compressor or a fan forcausing the heat transfer fluid to circulate around the dosed circuitand a heat extractor 34 which extracts heat from the heat transferfluid. In the illustrated embodiment, a circuit carries gaseous heliuminto and from the cryogen vessel 26. When inside the cryogen vessel, thehelium absorbs heat from the magnet structure and warms up. A compressoracting as circulator 32 compresses the gaseous helium to a certainpressure, typically in the range of 100-300 kPa absolute. Care must betaken not to pressurise the helium in excess of the capability of thecryogen vessel 26. The compressor causes the helium gas to travel aroundthe circuit and increases the density of the helium gas, therebyincreasing its capability for heat transfer. The compressed gas flowsfrom the compressor through closed pipes 36 into the cryogen tank 26.The helium absorbs heat from the magnet and is drawn through furtherpipes to heat extractor 34. The heat extractor may be an activecryogenic refrigerator such as a mechanical refrigerator. An example ofa mechanical refrigerator is one operating according to the Stirlingcycle. Alternatively, the heat extractor 34 may be a passiverefrigerator, such as a tank of liquid cryogen, or a mass of solidified,frozen, cryogen in thermal contact with the pipe 36 carrying the heattransfer fluid.

In a particular embodiment, a passive cooling arrangement employing atank of liquid cryogen or a mass of solid cryogen may be employed untilthe magnet has cooled to a first temperature, being no lower than thetemperature of the liquid or solid cryogen, with the flow of heattransfer fluid them being switched away from the liquid or solid cryogento an active refrigerator, to continue with cooling down to a desiredpre-cool temperature, lower than that which could be obtained from thepassive cooling arrangement alone.

As the magnet structure cools down, the density of the helium heattransfer fluid at a certain pressure will increase, increasing the heattransfer efficiency. If the heat extractor is sufficiently powerful, andparasitic thermal influx is kept to a minimum, the magnet willeventually be cooled to approximately its operating temperature.Alternatively, if the arrangement is not sufficiently powerful orefficient, the temperature of the magnet will stabilise. The gas in thepipes 36 and compressor 32 may even liquefy. At this point, the cryogenvessel may be filled with working cryogen. Since the working cryogen ispreferably used for the pre-cool cooling, there is no risk ofcontamination of the cryogen vessel with remnants of a sacrificialcryogen. Relatively little working cryogen is consumed in this process,since boiling of cryogen is used only to cool from one cryogenictemperature to the operating temperature, not for cooling from ambienttemperature.

Cooling is achieved either from electrical energy consumed by an activerefrigerator, or by boiling of a liquid cryogen, or by the melting of afrozen cryogen, or the heating or phase transition of any cooledcryogen.

Embodiments employing only electrically powered active coolingrefrigerators are the most portable.

While described with reference to helium, other cryogen will of coursebe used as appropriate for the material of the apparatus being cooled.

In embodiments as described with reference to FIG. 2, it should bepossible to cool known MRI system magnets at a rate of about 4K perhour, such that the magnet may be cooled from ambient temperature to atemperature of 4K in 74 hours. The efficiency of the heat transfersystem in removing heat from the magnet is limited by the mass flow rateof the heat transfer fluid. There are two alternatives for increasingthe mass flow rate. Firstly, the density of the fluid may be increasedby increasing the pressure of the gas; or the volume flow rate may beincreased. In the described embodiment, the pressure of the heattransfer fluid is applied to the interior of the cryogen vessel.Typically, the cryogen vessel can only withstand pressures of about 300kPa absolute. This limits the pressure which can be applied to the heattransfer fluid. Therefore, if it is necessary to increase the rate ofcooling by increasing the mass flow rate through the cryostat, this mustbe done by increasing the volume flow rate: the velocity of the heattransfer fluid through the pipes 36. This mass flow rate is determinedby the compressor 32. A fan may also be provided to assist withproviding the required volume flow rate. In certain embodiments, a fanmay be provided in place of the compressor. The fluid will circulate ata lower pressure, but its heat capacity will increase as it cools,leading to an effective cooling arrangement.

FIG. 3 schematically illustrates another embodiment of the presentinvention. In this invention, dual closed-loop cooling circuits areprovided. A first closed loop cooling circuit 50 acts to cool the magnet20 in a manner similar to that described with reference to FIG. 2,except in that the heat extractor is in a heat exchanger 42. Acirculator 52 is provided, to ensure a certain volume flow rate of firstheat transfer fluid around the circuit. The first heat transfer fluidflows into and out of the cryogen vessel 26, and so should be chosen tobe the same as the working cryogen to be employed within the cryogenvessel. Presently, this is most commonly helium. According to thisembodiment of the invention, a second closed loop cooling circuit 40cools the heat exchanger 42 by circulating a second heat transfer fluidthrough pipes between the heat exchanger 42 and a heat extractor 44. Theheat extractor may comprise an active refrigerator such as anelectrically powered cryogenic refrigerator, for example one operatingaccording to the Stirling cycle, or a passive heat extraction means suchas a tank of liquid cryogen, or a mass of frozen cryogen. In aparticular embodiment, illustrated, a tank of cryogen 46 is provided inparallel with a mechanical refrigerator 44, and operation of thisarrangement will be discussed in more detail below. The second heattransfer fluid need not be the same as the heat transfer fluid of thefirst closed loop cooling circuit 50. More particularly, it need not bethe same as the working cryogen to be employed in the cryogen vessel.

A particular advantage of the embodiment of FIG. 3 is that the pressureof the second heat transfer fluid employed in the second closed loopcooling circuit 40 is not limited by the pressure holding capabilitiesof the cryogen vessel 26. In operations the second closed loop coolingcircuit 40 may be brought into operation first, cooling the heatexchanger 42, before the magnet itself is available. In certainpreferred arrangements, it is considered advantageous to cool the heatexchanger 42 to a temperature of about 20K before commencing operationof the first cooling loop 50. Since such operation of the second loop 50is not constrained by the pressure limits of the cryogen vessel, anactive refrigerator 44 may be operated at its optimum pressure andefficiency. In such a method, when the first cooling loop 50 beginsoperation to cool the magnet, the first heat transfer fluid isimmediately cooled by the heat exchanger 42. This will increase theinitial density of a heat transfer fluid flowing to the magnet,increasing its mass flow rate, and will also increase the temperaturedifference between the magnet 20 and the heat exchanger 42. Each ofthese effects will increase the initial efficiency of cooling themagnet, thereby enabling effective cooling of the magnet to be completedin a shorter time. The heat exchanger 42 should be designed to have asignificant thermal mass, so that when cooling of the magnet begins, theheat exchanger will only warm slowly, keeping the rate of cooling themagnet relatively high and substantially constant.

In this embodiment, as with the embodiment of FIG. 2, the heatextraction may be performed using an active mechanical refrigerator 44.Alternatively, the second closed loop cooling circuit may be arranged topass in thermal contact with a low temperature thermal mass. Forexample, the pipes of the second closed loop cooling circuit may beplaced in contact with a bath of liquid nitrogen, to provide cooling toapproximately 70K. In another embodiment, the pipes of the second closedloop cooling circuit may be placed in contact with a mass of frozennitrogen, to provide cooling to significantly below 70K. In a moreadvanced version of such an embodiment, stainless steel pipes carryingthe heat transfer fluid may be embedded in a block of aluminum, and thewhole structure immersed in a bath or block of sacrificial cryogen. Forefficient cooling of the magnet, cooling may begin by circulation ofsecond heat transfer fluid around the second closed loop cooling circuit40 through a passive refrigeration means, such as a tank of liquidcryogen 46 or a block of solid cryogen. Once the heat exchanger 42 hasbeen cooled to the temperature of the liquid or solid cryogen, the flowof heat transfer fluid may be switched to flow to an active mechanicalrefrigerator 44 to allow further cooling of the heat exchanger 42 belowthe temperature of the tank of liquid cryogen 46 or block of solidcryogen. Should the temperature of the heat exchanger 42 rise above thetemperature of the tank of liquid cryogen 46 or block of solid cryogenagain, for example due to an influx of heat removed from the magnet 20,second heat transfer fluid may once again be allowed to flow through thetank of liquid cryogen 46 or block of solid cryogen, to cool the heatexchanger again.

Of course, the heat exchanger 42, the refrigerator 44, the tank ofliquid cryogen 46 and the pipes connecting these components must beeffectively thermally isolated to prevent thermal influx from thesurroundings. Similar considerations apply to embodiments such as shownin FIG. 2.

While the present invention has been described with reference to alimited number of particular embodiments, those skilled in the art willrecognise that various modifications and amendments may be made, withinthe scope of the invention as defined by the appended claims.

For example, electrically powered refrigerators operating according tothe Stirling cycle have been found to be very efficient and powerful inthe context of the present invention. (Such refrigerators have beenfound to be particularly compact, powerful and transportable). However,other types of cryogenic refrigerator are known, and could be employedin the present invention. The present invention has been described withparticular reference to helium as the working cryogen. While this isappropriate for conventional low-temperature superconducting magnets,other working cryogens may be employed within the scope of the presentinvention, according to the nature of the cryogenically cooledapparatus. For example, so-called high temperature superconductors areknown, and these may be cooled to a superconducting state by liquidnitrogen.

The heat exchanger discussed with reference to FIG. 3 may also beconsidered as a thermal battery: “cold” is stored in the heat exchanger,either by provision of a suitably cooled cryogen material or operationof the second closed loop cooling circuit. The stored “cold” is later'supplied” to the cooled equipment. The heat exchanger may beconstructed of any appropriate material. The material chosen should havea high thermal diffusivity and thermal capacity at the requiredtemperature of operation. Suitable materials need to be chosen for theheat exchanger according to the intended temperature of operation. Foroperation at a temperature of 20K, frozen nitrogen has been found to beappropriate. For operation at a temperature of 80K, water ice has beenfound effective. Both of these materials are abundant, inexpensive andnon-polluting.

Certain aspects of the present invention provide certain particularadvantages. By using a frozen block of cryogen as the secondary coolingsource, or the heat exchanger, cooling may be achieved to temperaturesbelow the boiling point of the cryogen used. For example, nitrogen maybe economically used as the cooling cryogen. Without further cooling,liquid nitrogen will cool to about 70K, by boiling at that steadytemperature. By initially cooling the cryogen, cooling to temperaturessuch as 20K may be effected, which in turn substantially reduces theamount of working cryogen needed to cool the magnet or other equipmentto its operating temperature. For example, using helium as an exampleworking cryogen, the use of boiling nitrogen for cooling the heattransfer fluid will require cooling from about 80K to about 4K byconsumption of liquid helium, whereas if the magnet or other equipmentcan be cooled to 20K, a much reduced quantity of liquid helium will berequired to cool from 20K to 4K.

Since the second cooling loop is not exposed to the interior of thecryogen vessel, the pressure of the second heat transfer fluid is notconstrained by the maximum pressure which the cryogen vessel canwithstand. For example, a typical cryogen vessel may have a maximumpressure capacity of 300 kPa absolute. The second closed-loop coolingcircuit may contain a gaseous cryogen pressurised significantly inexcess of the pressure of the heat transfer fluid of the firstclosed-loop cooling circuit. This increase of pressure significantlyincreases the heat transfer capacity of the fluid, by increasing itsdensity. Accordingly, the heat transfer capacity of the second coolingloop may be improved beyond that of the first closed-loop coolingcircuit, increasing the rate of cooling of the heat exchanger 42, and soalso the rate of cooling of the magnet or other equipment.

1. Apparatus for pre-cooling cryogenically cooled apparatus housedwithin a cryogen vessel, comprising a first closed-loop cooling circuitcontaining heat transfer fluid, a circulator for causing the heattransfer fluid to circulate around the closed circuit and a heatextractor arranged to extract heat from the heat transfer fluid, whereinthe circuit carries heat transfer fluid into and from an interior volumeof the cryogen vessel.
 2. Apparatus according to claim 11 wherein thecirculator comprises a compressor which acts to compresses a gaseousheat transfer fluid to a pressure in the range of 100-300 kPa absolute.3. Apparatus according to claim 11, wherein the heat extractor is anexternal mechanical active cryogenic refrigerator.
 4. Apparatusaccording to claim 1, wherein the heat extractor is a passive cryogenicrefrigerator comprising a reserve of a cryogen in thermal contact withthe closed-loop cooling circuit.
 5. Apparatus according to claim 4wherein the reserve of cryogen comprises a quantity of solid cryogen. 6.Apparatus according to claim 4, wherein the reserve of cryogen providescooling to a temperature of below 70K.
 7. Apparatus according to claim1, wherein the heat extractor comprises both an active cryogenicrefrigerator and a passive cryogenic refrigerators arranged such thatpassive cooling may be applied to the heat transfer fluid until thecryogenically cooled apparatus has cooled to a first temperature, withfurther cooling being provided by switching the flow of heat transferfluid to the active refrigerator, to continue with cooling down to adesired temperature, below that obtainable from the passive refrigeratoralone.
 8. Apparatus according to claim 1, wherein the circulatorcomprises a fan.
 9. Apparatus according to claim 1, wherein the heatextractor which extracts heat from the heat transfer fluid is a heatexchanger, itself cooled by a second closed-loop cooling circuitcontaining a second heat transfer fluid, a second circulator for causingthe second heat transfer fluid to circulate around the secondclosed-loop cooling circuit and a second heat extractor arranged toextract heat from the second heat transfer fluid.
 10. Apparatusaccording to claim 9 wherein the first and second heat transfer fluidsare both gases, and the second heat transfer fluid in the secondclosed-loop cooling circuit is at a higher pressure than the pressure ofthe heat transfer fluid in the first closed-loop cooling circuit. 11.Apparatus according to claim 9, wherein the second heat transfer fluidin the second closed-loop cooling circuit is of a different materialthan the heat transfer fluid in the first closed-loop cooling circuit.12. Apparatus according to claim 9, wherein the second heat extractor isan external mechanical active cryogenic refrigerator.
 13. Apparatusaccording to claim 9, wherein the second heat extractor is a passivecryogenic refrigerator comprising a reserve of a cryogen in thermalcontact with a pipe carrying the second heat transfer fluid around thesecond closed-loop cooling circuit.
 14. Apparatus according to claim 13wherein the reserve of cryogen comprises a quantity of solid cryogen.15. Apparatus according to claim 9, wherein the second heat extractorcomprises both an active cryogenic refrigerator and a passive cryogenicrefrigerators arranged such that passive cooling may be applied to thesecond heat transfer fluid until the cryogenically cooled apparatus hascooled to a first temperature, with further cooling being provided byswitching the flow of second heat transfer fluid to the activerefrigerator, to continue with cooling down to a desired temperature,below the temperature obtainable using the passive refrigerator alone.16. Apparatus according to claim 13, wherein the reserve of cryogenprovides cooling to a temperature of below 70K.
 17. Apparatus accordingto claim 9, wherein the second circulator comprises a fan.
 18. Apparatusaccording to claim 9, wherein the heat exchanger comprises a volume ofliquid or solid nitrogen; or water ice.
 19. A method for pre-cooling acryogenically cooled apparatus within a cryogen vessel, comprisingcirculating a heat transfer fluid through a first closed-loop coolingcircuit by operation of a circulator causing the heat transfer fluid tocirculate around the first closed-loop cooling circuit and extractingheat from the heat transfer fluid by use of a heat extractor in thermalconnection with the first closed-loop cooling circuit, wherein the heattransfer fluid flows into and from an interior volume of the cryogenvessel.
 20. A method according to claim 19 wherein the circulatorcomprises a compressor compressing gaseous heat transfer fluid to acertain pressure in the range of 100-300 kPa absolute.
 21. A methodaccording to claim 15, wherein heat extractor comprises an externalmechanical active cryogenic refrigerator.
 22. A method according toclaim 19, wherein heat extractor comprises a passive cryogenicrefrigerator, being a reserve of cryogen in thermal contact with thefirst closed-loop cooling circuit.
 23. A method according to claim 19,wherein heat extraction is initially performed by passive coolingemploying a reserve of cryogen until the cryogenically cooled equipmenthas cooled to a first temperature, being no lower than the temperatureof the sacrificial cryogen, with further heat extraction then beingperformed by an active refrigerator, to continue with cooling down to adesired pre-cool temperature.
 24. A method according to claim 19,wherein the heat extractor is a heat exchangers and the heat exchangeris itself cooled by a second closed-loop cooling circuit which cools theheat exchanger by circulating a second heat transfer fluid by operationof a circulator causing the second heat transfer fluid to circulatearound the second closed-loop cooling circuit and extracting heat fromthe second heat transfer fluid by use of a second heat extractor inthermal connection with the second closed-loop cooling circuit.
 25. Amethod according to claim 24 wherein the second closed loop coolingcircuit is brought into operation to cool the heat exchanger before thefirst closed loop cooling circuit is brought into operation.
 26. Amethod according to claim 24, wherein the second heat extractor is anexternal mechanical active cryogenic refrigerator.
 27. A methodaccording to claim 24, wherein the second heat extractor comprises apassive cryogenic refrigerator, being a reserve of cryogen in thermalcontact with the first closed-loop cooling circuit.
 28. A methodaccording to claim 24, wherein heat extraction by the second closed-loopcooling circuit is initially performed by passive cooling employing areserve of cryogen until the cryogenically cooled equipment has cooledto a first temperature, being no lower than the temperature of thereserve of cryogen, with further heat extraction then being performed byan active refrigerator, to continue with cooling down to a desiredpre-cool temperature, below the temperature obtainable by use of thereserve of cryogen alone.
 29. A method according to claim 24, whereinthe second circulator comprises a fan.
 30. A method according to claim24, wherein the first and second heat transfer fluids are both gases,and the second heat transfer fluid in the second closed-loop coolingcircuit is at a higher pressure than the pressure of the heat transferfluid in the first closed-loop cooling circuit.
 31. A method accordingto claim 24, wherein the second heat transfer fluid in the secondclosed-loop cooling circuit is of a different material than the heattransfer fluid in the first closed-loop cooling circuit.
 32. A methodaccording to claim 24, wherein heat extraction from the secondclosed-loop cooling circuit is performed by an external mechanicalactive cryogenic refrigerator.
 33. A method according to claim 24,wherein heat extraction from the second closed-loop cooling circuit isperformed by a passive cryogenic refrigerator comprising a reserve ofcryogen in thermal contact with the second closed-loop cooling circuit.34. A method according to claim 24, wherein heat extraction from thesecond closed-loop cooling circuit is performed by both an activecryogenic refrigerator and a passive cryogenic refrigerator, arrangedsuch that passive cooling is applied to the second heat transfer fluiduntil the cryogenically cooled apparatus has cooled to a firsttemperature, with further cooling being provided by switching the flowof second heat transfer fluid to an active refrigerator, to continuewith cooling down to a desired pre-cool temperature.
 35. A methodaccording to claim 24, wherein the second circulator comprises a fan.36. A method according to claim 24, wherein the heat exchanger is formedof a volume of liquid or solid nitrogen; or water ice.
 37. A methodaccording to claim 24, wherein the heat exchanger is cooled to a certaincryogenic temperature before operation of the first closed-loop coolingcircuit.
 38. Apparatus or a method according to claim 1, wherein in thefirst closed-loop cooling circuit, the circulator acts on cooled firstheat transfer fluid from the heat extractor on its way to the apparatusto be cooled.
 39. Apparatus or a method according to claim 1, wherein inthe second closed-loop cooling circuit, the circulator acts on cooledfirst heat transfer fluid from the second heat extractor on its way tothe heat exchanger.